<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    .first {
      width: 800px;
      height: 50px;
      background-color: aquamarine;
      margin: 50px auto;
      list-style: none;
      position: relative;
    }
    .first>li {
      float: left;
      width: 200px;
      height: 50px;
    }
    a {
      display: block;
      width: 100%;
      height: 100%;
      text-align: center;
      line-height: 50px;
    }

    div {
      width: 1400px;
      height: 400px;
      background-color: #ccc;
      position: absolute;
      top: 50px;
      left: -300px;
      display: none;
    }

    li:hover div {
      display: block;
    }
  </style>
</head>
<body>
  <ul class="first">
    <li>
      <a href="#">首页</a>
      <div>首页的下拉</div>
    </li>
    <li>
      <a href="#">产品中心</a>
      <div>产品中心的下拉</div>
    </li>
    <li>
      <a href="#">联系我们</a>
      <div>联系我们的下拉</div>
    </li>
    <li>
      <a href="#">产品中心</a>
      <div>产品中心的下拉</div>
    </li>
  </ul>
</body>
</html>